One of the most challenging and formidable tasks in preparative organic chemistry is the selective functionalization of a carbon-hydrogen bond. Once a functional group has been introduced onto a carbon, the chemist has a rich selection of tools to achieve further transformations and transpositions, but it is clear that the initial barrier of introducing a functional group is determinative of further chemistry.
It is not only necessary that a given functionalization reaction proceed in good yield, but it also is necessary that it proceeds with specificity. Consider, for example, the reaction of simple hydrocarbons with oxygen which has multiple reaction paths including, ##STR1## as well as numerous other products arising from carbon-carbon bond cleavage. It is clear that it will be only the rare case where such indiscriminate oxidation will be a useful process for functionalization of a carbon-hydrogen bond.
Thus recent disclosures that certain titanium-containing molecular sieves are reasonably selective and active in the oxidation of an aromatic to a hydroxy-aromatic compound according to the equation, EQU Ar--H.fwdarw.Ar--OH
where Ar is an aromatic moiety, are particularly noteworthy. We now proceed to briefly review this art not only to determine its precise content but also to clearly distinguish our invention from the prior art teachings.
In U.S. Pat. No. 4,396,783 the patentees disclosed the oxidation of phenols to hydroquinone and pyrocatechol by hydrogen peroxide at 80.degree.-120.degree. C. as catalyzed by a crystalline silica (silicalite) modified by introduction into the molecular sieve of a metal such as chromium, beryllium, titanium, vanadium, manganese, iron, cobalt, zinc, zirconium, rhodium, silver, tin, antimony, and boron. The reaction can be heterogeneous or it can be conducted in the presence of the solvent such as water, methanol, acetone, isopropyl alcohol, and acetonitrile, all of which provide at least partial miscibility with hydrogen peroxide. The patentees also reported that in addition to phenols the reaction proceeded on substrates such as toluene, anisole, xylenes, mesitylene, benzene, nitrobenzene and ethylbenzene. Although the yield of hydroxylated products from phenol and anisole are on the order of 50%, no results were given for benzene. It is significant that the yields for the hydroxylation of toluene generally were about 20%, although there were two examples where the yield was approximately 40%.
Taramasso et al. in U.S. Pat. No. 4,410,501 describe titanium silicalites (TS-1) where the atom ratio of titanium to silicon was in the range of 0.0005-0.04, and in GB 2,116,974 they described their use in the oxidation of phenols to diphenols. Where the oxidation was done at a high molar ratio of phenol to hydrogen peroxide (i.e., a large excess of phenol) and at 80.degree.-120.degree. C., the utilization of hydrogen peroxide (given by the patentees as "H.sub.2 O.sub.2 yield") was high, approaching 90%, but as the mole ratio decreased the peroxide utilization dropped to about 60% accompanied by a decrease in phenol selectivity. The reactivity of TS-1 in catalyzing the oxidation of various organics by hydrogen peroxide was summarized in La Chimica and L'Industria, 72-1990, pages 610-616, where phenol hydroxylation to hydroquinone and catechol was reported at some length. Pertinent to the discussion here is the observation that yields based on converted hydrogen peroxide were in the order of 80% at 100% hydrogen peroxide conversion when the reaction was performed at reflux in water, in methanol, and in 60:40 water:acetone and at a peroxide:phenol molar ratio of 0.25-0.35 and the peroxide was in the form of 30% aqueous hydrogen peroxide. Thus, even with a large excess of phenol, conversions were modest.
A. Thangaraj et al., J. Applied Catalysis, 57, (1990), L1-L3, studied the oxidation of benzene by hydrogen peroxide to phenol and benzoquinone as catalyzed by various zeolites and molecular sieves. Of particular importance to this application is the observation that whereas the selectivity of hydrogen peroxide utilization to oxygenated aromatics decreased in the order, EQU TS-1&gt;Fe-TS-1&gt;Al-TS-1&gt;Fe-ZSM-5&gt;Al-ZSM-5,
the selectivity to phenol vis a vis total oxidized benzenes increased in the same order. Stated differently, TS-1 was the most effective material in the utilization of H.sub.2 O.sub.2 in forming oxidized benzenes, but it afforded the greatest product spectrum. What is also significant is the observation that an aluminum-containing TS-1 (Si/Al=86, Si/Ti=24) afforded only 37% selectivity in hydrogen peroxide decomposition with respect to hydroxy benzenes formation. It is particularly significant to observe that the zeolites were freed of sodium ions prior to use. Particularly relevant is the statement by Notari based on experimental data [Structure-activity and Selectivity Relationships in Heterogeneous Catalysis, R. K. Grasselli and A. W. Sleight, Editors, 1991, Elsevier Science Publishers B. V., Amsterdam] that the yield of hydrogen peroxide (that is, the molar percentage of hydrogen peroxide decomposition which leads to phenolic products) decreases with the addition of sodium or potassium ions to TS-1, where the magnitude of the decrease is a function of the amount of alkali metal added. For example, the addition of 7060 ppm potassium ion reduced the hydrogen peroxide yield from 79.5 to 0, and the addition of 3529 ppm sodium ion reduced the peroxide yield from 79.5 to 22.
Tuel and coworkers studied the solvent effects in phenol oxidation by hydrogen peroxide as catalyzed by TS-1 [J. Molec. Catalysis, 68, (1991), 45-52]. Another titanium silicalite, TS-2, with a MEL structure recently was synthesized by Reddy and Kumar, Jour. of Catalysis, 130, 440-6 (1991), who observed it to be catalytically active in the oxidation of benzene by 26% hydrogen peroxide. Although the predominant product was phenol, para-benzoquinone was produced in over 30% yield.
A fair summary of the prior art TS-1 titanium silicalite is that it is active in the oxidation of phenol at 80.degree.-120.degree. C., especially with a solvent in the absence of alkali metals. Benzene itself is considerably more difficult to oxidize and is oxidized far less selectively than phenol since significant amounts of benzoquinone accompany phenol formation. Incorporation of aluminum into the framework adversely effects hydrogen peroxide utilization efficiency, and a concentration of peroxide of at least 26% is necessary as an oxidizing agent. A high molar ratio of aromatic to hydrogen peroxide usually is necessary to obtain high (at least 90%) utilization of hydrogen peroxide, but decreasing the ratio generally led to a dramatic decrease in hydrogen peroxide utilization. That benzene is much less active than phenol is not surprising in view of the generally activating effect the hydroxyl group has on an aromatic ring.
Although it is promising start, the results with TS-1 are inadequate for a commercial process of oxidation of, for example, benzene to phenol. Any commercial process would require that dilute peroxides would be sufficient--something on the order of 5% hydrogen peroxide instead of the prior art's 26-30% hydrogen peroxide. A commercial process also requires that approximately equal molar amounts of benzene and hydrogen peroxide be used with high efficiency of hydrogen peroxide utilization, i.e., approximately 90%. A commercially feasible process also would require a high conversion of the aromatic, such as benzene, above about 80%, but above all one with a high selectivity to the hydroxy aromatic, e.g., phenol. We have found that certain aluminosilicates having titanium in the framework, and especially those which have been exchanged with an alkali or alkaline earth metal cation, are effective in oxidizing aromatics to hydroxyaromatics using hydrogen peroxide, especially in dilute (i.e., under 10%) solutions. It is particularly noteworthy that the prior art teaches that both incorporation of aluminum into the zeolite and the presence of an alkali metal cation are detrimental to the catalytic effectiveness of a titanium-zeolite in oxidizing aromatics by hydrogen peroxide. Accordingly, one would hardly suspect that our formula for success would be advantageous.